The present invention relates generally to gas turbine engines and more particularly to an improved hollow fan blade for a gas turbine engine.
A gas turbine engine, such as a turbo fan engine for an aircraft, includes a fan section, a compression section, a combustion section and a turbine section. An axis of the engine is centrally disposed within the engine and extends longitudinally through the sections. The primary flow path for working medium gases extends axially through the sections of the engine. A secondary flow path for working medium gases extends parallel to and radially outward of the primary flow path.
The fan section includes a rotor assembly and a stator assembly. The rotor assembly of the fan includes a rotor disc and plurality of radially extending fan blades. The fan blades extend through the flow path and interact with the working medium gases and transfer energy between the fan blades and working medium gases. The stator assembly includes a fan case, which circumscribes the rotor assembly in close proximity to the tips of the fan blades.
During operation, the fan draws the working medium gases, more particularly air, into the engine. The fan raises the pressure of the air drawn along the secondary flow path, thus producing useful thrust. The air drawn along the primary flow path into the compressor section is compressed. The compressed air is channeled to the combustion section where fuel is added to the compressed air and the air/fuel mixture is burned. The products of combustion are discharged to the turbine section. The turbine section extracts work from these products to power the fan and compressed air. Any energy from the products of combustion not needed to drive the fan and compressor contributes to useful thrust.
In order to reduce weight, the fan blades in some gas turbine engines are hollow. Each fan blade is made by combining two separate detail halves. Each half includes a plurality of cavities and ribs machined out to reduce the weight while forming a structurally sound internal configuration. One half forms the pressure side wall and the other half forms the suction side wall. When the detail halves are joined, the pressure side wall and the suction side wall are separated and supported by the ribs to form the hollow fan blade. The hollow fan blade is then subjected to forming operations at extremely high temperatures at which time it is given an airfoil shape and geometry. The side walls are contoured and curved to form the airfoil.
Fan blades must be capable of withstanding the impact of birds, ice or other foreign objects. These apply extreme initial loads at the leading edge, tending to cause bending of the airfoil at the leading edge, which applies a large compressive load to the suction side cavity walls. For this load, spanwise ribs and cavities offer the least resistance to buckling, and it's for this reason that advanced blade configurations feature outer span ribs that run chordwise. There are also secondary loads milliseconds after the impact as a shock wave radiates out from the impact site. These secondary loads flex the metal as they are passed onward. Subsequently, the blade tip will flex back and forth until the energy from the event can be absorbed. These motions alternate compressive and tensile loads to both pressure and suction cavity walls. Blade tips are also subject to circumferential loading when blades rub the case during heavy crosswinds or maneuver loading, or in a severe case, during a blade out event. Tip rubs apply similar compressive loads to the suction side cavity walls.
Compressive loads on the walls between ribs can cause the walls to buckle, depending on the wall thickness and span and other geometry. These considerations increase the minimum wall thickness that must be used. This increases the blade weight, and with hub and containment considerations, the total engine weight.